The role played by the primary somatosensory cortex (SI) in the perception of painful stimuli is poorly understood. We have used two approaches to resolve this problem. In the first, SI neuronal activity was correlated with the ability of monkeys to detect small increments in noxious thermal stimulation. In the second approach, the ability of the monkeys to detect and to discriminate noxious thermal stimulation was examined during a reversible lesion, produced by cooling the somatosensory cortex. The activity of nociceptive SI neurons was recorded while the monkey performed a psychophysical task. The monkeys detected small (0.2 degrees -0.8 degrees C) increases in ski temperatures superimposed on noxious levels of thermal stimulation (45-49 degrees C) applied to the face. The detection latency, expressed as detection speed, was used as a measure of the perceived intensity of sensation. Two-thirds o the neurons that responded to noxious thermal stimulation increased their discharge rate with increases in stimulus intensity. The remaining neurons responded to noxious thermal stimulation, but did not grade their discharge rates with increases in stimulus intensity. The neuronal discharge of nociceptive SI neurons was correlated with the monkeys' ability to detect noxious thermal stimulation. A significant correlation was found between the peak neuronal discharge and the monkeys' detection speed. In addition the discharge of nociceptive SI neurons was significantly greater on correctly detected vs non-detected trials. Increases in foreperiod length produced an increase in both the detection speed of the monkey as well as an increase in the peak neuronal discharge of nociceptive SI neurons. In the cortical cooling experiments, the monkeys were required to detect small increments in noxious thermal stimulation (detection task). On some trials the monkeys were required to discriminate that the thermode temperature was higher than on detection trials, to release the button and to escape the noxious thermal stimulus. After cooling SI, the monkey exhibited clear deficits in the detection and the discrimination tasks. We conclude that nociceptive SI neurons are involved in the process through which monkeys perceive the intensity of noxious thermal stimulation.